High power silica fiber lasers are used for material processing (cutting welding) and numerous other applications. In principle, the fibers are made of a core and clad. The core is doped with rare earth ions for lasing media.
Light is typically transmitted by means of total reflection due to a difference of optical refractive indices between the clad and the core. Both the fiber core and cladding are typically doped elements such as germanium dioxide (GeO2) or aluminium oxide (Al2O3)) are added to the silica core to slightly increase the optical refractive index. The aluminium also plays a role of making the rare earth ions soluble in the SiO2. Fluorine or boron trioxide (B2O3) is used to lower the refractive index.
Ge is also used for UV writing of channel waveguides. The core is doped with an amount of 4.1 wt. % Ge and 12.3 wt. % of phosphorous.
In general, the process of fabricating the optical fiber is further divided into a process of fabricating an optical fiber preform and a process of drawing an optical fiber from the optical fiber preform. The process of fabricating an optical fiber preform is accomplished by utilizing such methods as a modified chemical vapor deposition (MCVD) method, or other methods such as a vapor-phase axial deposition (VAD) method and an outside vapor deposition (OVD) method, coupled with solution doping. In the process of MCVD, a porous core layer with appropriate thickness and composition is deposited over the cladding layer followed by soaking with an aqueous/alcoholic solution of suitable composition for a fixed time span. Subsequent to solution doping, the soaked layer is dried and sintered for transformation to clear glass. The tube is finally collapsed to a solid rod known as a preform.
Fiber of standard dimension is drawn from the preform using a fiber drawing tower
Planar waveguide lasers have potential advantages over the fiber lasers: standard microelectronics production techniques, heat removal and pumping schemes. Waveguide fabrication does not require fiber drawing, which requires non-standard, costly equipment and brings limitations to the possible designs.
There are a number of techniques to produce silica waveguides such as thermal oxidation of silicon crystals, deposition of SiO2 thin films by sputtering or thermal evaporation, ion exchange in silica substrates and sol-gel preparation.
For high power laser applications the waveguides have to be relatively thick (e.g., 10-100 microns), highly doped with the rear earth ions and of very high optical quality. None of the above mentioned techniques has the ability to produce a waveguide with the required high power properties. Thus high power waveguide lasers have not been realized until now.